1. Field of the Invention
The present invention relates to a frequency offset detector and method for detecting the frequency offset of a reception signal in, for example, integrated services digital broadcasting-terrestrial (ISDB-T).
2. Description of the Related Art
A broadcast signal of ISDB-T is constituted of 13 orthogonal frequency division multiplex (OFDM) segments (hereinafter simply referred to as “segments”) in the case of television broadcasting and one to three segments in the case of radio broadcasting. One segment is a packet of a predetermined number (e.g., 108 in Mode 1) of carrier waves corresponding to a transmission mode, and has a band of about 430 kHz. The carrier waves include a control information carrier modulated by a predetermined modulation method, and a data carrier which is modulated by a modulation method indicated by the control information carrier and which transmits the main information of a broadcast.
In one segment, each carrier wave is modulated by an individual complex symbol (a so-called IQ symbol which indicates an orthogonal component of an information signal with a real part and an imaginary part) at every symbol period (modulation period: about 1 ms), and multiplexed into one OFDM symbol and transmitted. Thus, 204 OFDM symbols constitute one transmission frame.
FIG. 2 is a diagram showing an example of the configuration of the transmission frame of the ISDB-T. In FIG. 2, there are shown carriers that are arranged from left to right in ascending order of frequencies, and OFDM symbols that are arranged from the top to bottom in order of time. One complex symbol c(n, k) for modulating a carrier k during the period of a symbol number n is positioned in a cell where the carrier and the OFDM symbol intersect with each other. Therefore, this diagram shows the arrangement of the carriers of the complex symbols c(n, k) in order of frequency and time.
Symbols marked “SP” in FIG. 2 represent SP symbols which are pilot symbols indicating reference values used for the equalization of signals. The SP symbol is transmitted by one out of three carriers once in four symbol periods in order of time. Moreover, the SP symbol is transmitted by one out of twelve carriers in all the symbol periods in order of frequencies.
Furthermore, the symbol marked “TMCC” in FIG. 2 transmits a transmission and multiplexing configuration control (TMCC) signal using the predetermined control information carrier. The TMCC signal includes synchronization symbols indicating the synchronization timing of the frames in symbol numbers 1 to 16, segment format identification symbols in symbol numbers 17 to 19, and TMCC information symbols indicating the kind of the segment, the modulation method, etc. in symbol numbers 20 to 121. In addition, the control information carrier is set to be modulated by a differential binary phase shift keying (DBPSK) method. Moreover, symbols which are not marked either “SP” or “TMCC” in FIG. 2 are data symbols for transmitting the main information of a broadcast.
While such a broadcast signal is transmitted as a radio signal RF of 450 to 700 MHz, a phenomenon called a multipath occurs wherein radio waves reflected by buildings and walls and traveling through various paths reach a receiving end in addition to radio waves directly reaching a receiving antenna from an antenna of a sending end. If the multipath occurs, the reflected radio waves arrive later than the radio waves directly reaching the receiving antenna from the sending antenna, so that a signal coming later overlaps a signal coming first and it is difficult to accurately receive the signals. Thus, in the OFDM method, a guard interval is used to prevent the overlap of the signals.
Specifically, a large number of carrier waves are used to simultaneously send data in the OFDM method, and when each carrier wave carries data, the whole second half of a signal modulated from the data is copied and attached to the head of this signal and thus sent instead of sending the signal modulated from the data as it is. This copied part is the guard interval. The attachment of the guard interval to the original signal makes one symbol longer, but a part with no overlap long enough to extract the original signal remains owing to redundancy corresponding to the guard interval even if the signal coming later overlaps the signal coming first to some extent at the time of reception due to the occurrence of the multipath. This can minimize the effect of the multipath. Moreover, this guard interval is also used to correct a reception frequency.
FIG. 3 is a diagram of the configuration of a frequency correcting system in a conventional OFDM receiver. In this frequency correcting system, a received input signal IN is provided to a frequency conversion circuit 1, and converted to an intermediate frequency signal IF by an automatic frequency control signal AFC and then output to an autocorrelation detector 2. The autocorrelation detector 2 finds the autocorrelation function of the intermediate frequency signal IF, and this autocorrelation function is provided to a frequency controller 3. On the basis of the autocorrelation function, the frequency controller 3 generates an automatic frequency control signal AFC such that the intermediate frequency signal IF has a predetermined frequency, and the automatic frequency control signal AFC is fed back to the frequency conversion circuit 1. Thus, the intermediate frequency signal IF output from the frequency conversion circuit 1 is controlled so that it has the predetermined frequency. In addition, the intermediate frequency signal IF controlled to have the predetermined frequency is provided to a fast Fourier transform circuit (hereinafter referred to as “FFT”) 4, and transformed into a reception signal R(n, k) converted to a signal for each carrier wave forming a segment.
The frequency correcting systems, described above, have been utilized throughout the prior art. For example, Japanese Patent Publication Laid-open No. 2004-153811 provides a discussion of broadcast signals including a disclosure of television broadcasting and radio broadcasting segments wherein a frequency correcting system is described therein. Another exemplary frequency correcting system is disclosed by Japanese Patent Publication Laid-open No. 2005-45664.
However, in the frequency correcting systems described above, the guard interval provided for each symbol is used to generate the automatic frequency control signal AFC. This makes it impossible to make a correction when one or more rotations (corresponding to the frequency of one carrier wave) are produced by the frequency offset within one effective symbol. That is, if the frequency of the intermediate frequency signal IF is shifted by the frequency offset, the SP symbol is shifted from a regular carrier number 0 to a carrier number 1 or carrier number 2 as shown in FIG. 4, so that no SP symbol is contained in the reception signal of the carrier number 0 output from the FFT 4 and data can not be normally received.
Although the frequency correction systems discussed hereinabove provide a degree of automatic frequency control, further improvement to correct frequency offset is desirable.